Method for writing a planar waveguide having gratings of different center wavelengths

ABSTRACT

Multiple Bragg gratings are fabricated in a single planar lightwave circuit platform. The gratings have nominally identical grating spacing but different center wavelengths, which are produced using controlled photolithographic processes and/or controlled doping to control the effective refractive index of the gratings. The gratings may be spaced closer together than the height of the UV light pattern used to write the gratings.

FIELD OF THE INVENTION

[0001] The present invention relates to waveguide gratings and moreparticularly to a method for writing waveguide gratings having differentcenter wavelengths.

BACKGROUND INFORMATION

[0002] An optical telecommunication system transmits information fromone place to another by way of a carrier whose frequency is in thevisible or near-infrared region of the electromagnetic spectrum. Such acarrier is sometimes referred to as an optical signal, an opticalcarrier, or a lightwave signal. Optical fibers transport the lightwavesignal, each of which includes several channels. A channel is aspecified frequency band of an electromagnetic signal, and is sometimesreferred to as a wavelength. Multiple channels are commonly transmittedover the same optical fiber to take advantage of the unprecedentedcapacity offered by optical fibers. Essentially, each channel has itsown wavelength, and all wavelengths are separated enough to preventoverlap. Typically, hundreds or thousands of channels are interleaved bya multiplexer, launched into the optical fiber, and separated by ademultiplexer at a receiver. Along the way, channels may be added ordropped using add/drop multiplexers (ADM) or switched using opticalcross-connects (OXC).

[0003] Wavelength division multiplexing (WDM) facilitates propagation ofmultiple channels in a single optical fiber. Wavelength divisiondemultiplexing elements separate the individual wavelengths usingfrequency-selective components such as optical gratings, which canprovide high reflectivity and high wavelength selectivity with the aimof increasing the transmission capacity of optical fibers. One suchoptical grating is a Bragg grating (e.g. in fiber or in planarwaveguides), which selectively transmits or reflects specificwavelengths of light propagating within the optical fiber.

[0004] A Bragg grating is a portion of an optical fiber or planarwaveguide that has a refractive index profile, which varies periodicallyalong the length of the optical fiber. The center wavelength profile ofa Bragg grating is determined by the following equation:

λ=2nΛ  (Equation 1)

[0005] where λ is the center (or Bragg) wavelength, n is the meaneffective refractive index, and Λ is the period of the grating (orgrating spacing).

[0006] Simple periodic fiber Bragg gratings are known in the art andmany different methods have been described for fabricating fiber Bragggratings. One characteristic of fiber Bragg gratings is that, asEquation 1 indicates, to change the center wavelength profile, one canchange the refractive index or the grating spacing. Prior art techniquesfocus on changing the grating spacing, which is accomplished by changingthe interference pattern used to define the grating profile. Theinterference pattern is changed by changing the inter beam angle betweentwo overlapping interfering ultraviolet (UV) light beams used to exposethe optical fiber or by changing a phase mask through which UV light isshined.

[0007] Changing the phase mask or the inter beam angle tends to beexpensive, cumbersome, and labor intensive, however, especially whentrying to fabricate several different types of fiber Bragg gratings forthe myriad filtering and other applications in optical communicationsystems. For example, to fabricate fiber Bragg gratings with differentcenter wavelength profiles the writing apparatus is set to differentwavelengths, currently by replacing the phase masks. To write long fiberBragg gratings the optical fiber is translated on long-travel stages toexpose new portions of the photosensitive optical fiber to UV light.Similarly, to write chirped broadband fiber-based gratings chirped masksare generally used and new phase masks are used for each new chirpprofile. Additionally, writing individual Bragg gratings into separateoptical fibers commonly requires time consuming multiple exposures andextensive handling of optical fibers to control the optical fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In the drawings, like reference numbers generally indicateidentical, functionally similar, and/or structurally equivalentelements. The drawing in which an element first appears is indicated bythe leftmost digit(s) in the reference number.

[0009]FIG. 1 is a schematic diagram of a photonic device according to anembodiment of the present invention.

[0010]FIG. 2 is a flowchart illustrating an approach to fabricating thephotonic device in FIG. 1 according to embodiments of the presentinvention.

[0011]FIG. 3 is a schematic diagram of a photonic device according to analternative embodiment of the present invention.

[0012]FIG. 4 is a graph illustrating an approach to fabricating thephotonic device in FIG. 3 according to embodiments of the presentinvention.

[0013]FIG. 5 is a schematic diagram of a photonic device according toanother embodiment of the present invention.

[0014]FIG. 6 is a flowchart illustrating an approach to fabricating thephotonic device in FIG. 5 according to embodiments of the presentinvention.

[0015]FIG. 7 is a schematic diagram of a photonic device according to anembodiment of the present invention.

[0016]FIG. 8 is a flowchart illustrating a process for making thephotonic device illustrated in FIG. 7 according to embodiments of thepresent invention.

[0017]FIG. 9 is a high-level block diagram of a system for makingphotonic devices according to embodiments of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0018] Embodiments of the present invention are directed to fabricationof waveguide gratings. In the following description, numerous specificdetails, such as particular processes, materials, devices, and so forth,are presented to provide a thorough understanding of embodiments of theinvention. One skilled in the relevant art will recognize, however, thatthe embodiments of the present invention can be practiced without one ormore of the specific details, or with other methods, components, etc. Inother instances, well-known structures or operations are not shown ordescribed in detail to avoid obscuring the understanding of thisdescription.

[0019] Some parts of this description will be presented using terms suchas wavelength, silicon, taper, grating, chirp, and so forth. These termsare commonly employed by those skilled in the art to convey thesubstance of their work to others skilled in the art.

[0020] Various operations will be described as multiple discrete blocksperformed in turn in a manner that is most helpful in understandingembodiments of the invention. However, the order in which they aredescribed should not be construed to imply that these operations arenecessarily order dependent or that the operations be performed in theorder in which the blocks are presented.

[0021] Reference throughout this specification to “one embodiment” or“an embodiment” means that a particular feature, structure, process,block, or characteristic described in connection with the embodiment isincluded in at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

[0022]FIG. 1 is a schematic diagram of a photonic device 100 accordingto an embodiment of the present invention. The photonic device 100includes a single waveguide 102 formed in or on a planar lightwavecircuit (PLC) platform 104. The waveguide 102 includes several cascadedgratings 106, 108, and 110.

[0023] The waveguide 102 may have a circular pattern (as depicted inFIG. 1) or other layout. The waveguide 102 may be a single modewaveguide. Alternatively, the waveguide 102 may be a multimodewaveguide.

[0024] The PLC platform 104 may be any suitable PLC platformmanufactured using appropriate semiconductor processing equipment. Forexample, the platform 102 may be a silica-on-silicon platform, a lithiumniobate (LiNbO₃) platform, a gallium arsenide (GaAs) platform, an indiumphosphide (InP) platform, a silicon-on-insulator (SOI) platform, asilicon oxynitride (SiON) platform, a polymer platform, or othersuitable planar lightwave circuit (PLC) platform.

[0025] The gratings 106, 108, and 110 may be Bragg gratings whosegrating spacing (Λ) are nominally identical but whose center wavelengthare different because each grating 106, 108, and 110 is doped with adifferent concentration and/or type of dopant to give the gratingregions of the waveguides 106, 108, and 110 different effectiverefractive indices after being written. The grating regions of thewaveguides are the locations where gratings will be written. The dopantmay be any suitable photosensitive material, such boron (B), germanium(Ge), and/or phosphorous (P). The refractive index of such a dopedregion will change depending on the UV “dosage”. Thus, if the UV lighthas a periodically varying intensity pattern (as is the case whenwriting a grating), after UV exposure the doped region will have aperiodically varying refractive index, thereby forming a grating.

[0026] In another embodiment, the refractive indices may be modulatedlocally through hydrogenating the sample and pre-exposing selectedsections with UV light. After hydrogen out-gassing, these sectionsremain photosensitive. The UV dosage controls the average refractiveindex as well as the induced photosensitivity. On example ofhydrogenating is described in more detail below in conjunction with FIG.3.

[0027] The gratings 106, 108, and 110 may be close together (e.g.,spacing 120 can be less than the height of the UV light intensitypattern used to write the gratings 106, 108, and 110). By arranging thegratings closely together, the gratings may be written in one exposurein some embodiments.

[0028] One or more of the gratings 106, 108, and 110 may be longer thanthe length of the UV light beam used to write the gratings. In oneembodiment of the present invention, the UV light beam is one centimeterlong and the grating 106 is two centimeters long. In other embodiments,the dimensions of the UV light beam and/or the gratings may bedifferent.

[0029]FIG. 2 is a flowchart illustrating a process 200 for making thephotonic device 100 (FIG. 1) according to embodiments of the presentinvention. A machine-readable medium with machine-readable instructionsthereon may be used to cause a processor to perform the process 200. Ofcourse, the process 200 is only an example process and other processesmay be used. The order in the process 200 are described should not beconstrued to imply that these operations are necessarily order-dependentor that the operations be performed in the order in which the blocks arepresented.

[0030] Referring to FIGS. 1 and 2, an operation 202 is performed tofabricate the waveguide 102 in or on a PLC platform using standardsemiconductor fabrication techniques. These fabrication techniques caninclude implantation, doping, evaporation, physical vapor deposition,ion assisted deposition, photolithography, magnetron sputtering,electron beam sputtering, masking, reactive ion etching, and/or othersemiconductor fabrication techniques well known to those of ordinaryskill in the art. For example, in one embodiment, the waveguide 102 hasa core formed from oxide such as silica.

[0031] Still referring to FIGS. 1 and 2, an operation 204 is performedto dope selected regions of the waveguide 102 that will serve as thegratings. For example, a temporary mask layer can be formed on thewaveguides, which is then patterned to define the grating regions in thewaveguides. This process can be performed before the top cladding layeris deposited. In some embodiments, this process can be performed afterthe top cladding layer is formed so that the top cladding layer can bedoped instead of the core layer. In this alternative embodiment, theevanescent field of the propagating light can be affected by a gratingwritten in cladding layer. Alternatively, the cladding can be patternedand etched to define the grating regions, with the cladding layer itselfserving as a mask for the doping process. The grating regions are thenselectively doped (described below).

[0032] The grating regions are selectively doped with a predeterminedconcentration or a predetermined composition of one or morephotosensitive materials. In some embodiments, the dopants include boron(B), germanium (Ge), and/or phosphorous (Ph). The grating regions can bedoped using any suitable doping processes such as, for example, ionimplantation, diffusion from spin-on solutions, or other current orfuture techniques.

[0033] In one embodiment, the region of waveguide 102 corresponding tothe grating 106 is doped with a first predetermined concentration ofgermanium, which gives the grating 106 a first refractive index and afirst center wavelength. Similarly, the region of waveguide 102 regioncorresponding to the grating 108 is doped with a second predeterminedconcentration of germanium, which gives the grating 108 a secondrefractive index and a second center wavelength. Likewise, the region ofwaveguide 102 corresponding to the grating 110 is doped with a thirdpredetermined concentration of germanium, which gives the grating 110 athird refractive index and a third center wavelength. In one embodimentof the present invention, dopant concentrations may vary the refractiveindex of the waveguide regions for the gratings 106, 108, and 110 byapproximately 0.2 percent to shift the center wavelength by 0.2 percentor 30 nm at 1550 nm. Gratings can then be written in doped regions ofwaveguide 102 (FIG. 1) as described below.

[0034] Again referring to FIGS. 1 and 2, an operation 206 is performedto expose one or more of the doped regions of waveguide 102(corresponding to the gratings 106, 108, and/or 110) to an UV lightintensity pattern in a direction transverse to the waveguide 102. ThisUV light intensity pattern determines the grating spacing for thegratings 106, 108, and 110. In one embodiment, the exposure writes allgratings simultaneously so that the gratings 106, 108 and 110 all havethe substantially the same grating spacing. In other embodiments, eachof the gratings may be written individually, or any subset of thegratings may be written simultaneously.

[0035] In one embodiment, a suitable KrF excimer laser/phase mask unitis used to expose the doped regions of waveguide 102 (FIG. 1)corresponding to the gratings 106, 108, and 110 to a selected UV lightintensity pattern. Because the regions were doped differently from eachother, the resulting waveguide gratings will generally have differentcenter wavelengths even though the grating spacing is substantiallyidentical. In one embodiment, the grating 106 has a center wavelength of1555 nm, the grating 108 has a center wavelength of 1520 nm, and grating110 has a center wavelength of 1560 nm.

[0036] For example, a KrF excimer laser/phase mask unit can beconfigured to output the UV light intensity pattern with a height of 300microns. In one embodiment, the regions of waveguide 102 for thegratings 106, 108, and 110 are arranged so that the total area occupiedby the doped regions for gratings 106, 108 and 110 has a height that isless than 300 microns. The resulting photonic device is more compact andallows portions of all three gratings to be written simultaneously in asingle exposure. Additionally, no wide-bandwidth tuning of theindividual exposure (interbeam angle) is needed to tune the waveguidegratings to a center wavelength.

[0037] In an embodiment of the present invention in which one or more ofgratings is to be longer than the length of an UV light beam used toexpose the gratings, the region of the waveguide for the grating may befolded such that the entire grating can be written in the same exposure.For example, in embodiments in which the UV light beam is onecentimeter, the grating may be two centimeters long but be folded intoone-centimeter (or less) portions. In this case, the resulting gratinghas gaps (i.e. sections of waveguide where there is no grating written)between the grating segments. Such devices are known as sampled orsegmented Bragg gratings.

[0038] The photonic device 100 may be implemented as a waveguide filterto compensate for chromatic dispersion. Chromatic dispersion is thetemporal separation of the constituent colors of a lightwave pulse andcan be problematic because it causes channel amplitudes to vary andcauses adjacent data bits or pulses to bleed into each other, causinginter-symbol interference (ISI). In an optical fiber, dispersion occursbecause different wavelengths propagate at different speeds.Compensation for dispersion in optical fiber networks becomesincreasingly important as bit rates increase because the bits (opticalpulses) are now spaced closer together and the shorter pulses contain alarger span of bandwidth.

[0039] The photonic device 100, when operating, receives a multiplexedlightwave signal, which enters the waveguide 102 and is incident on thegratings 106, 108, and 110. The multiplexed lightwave signal has severalsingle channel lightwave signals each with its own center wavelengthprofile. According to embodiments of the present invention, the grating106 reflects a first wavelength (e.g., 1535 nm) and passes otherwavelengths, the grating 108 reflects second wavelength (e.g., 1550 nm)and passes other wavelengths, and the grating 110 reflects a thirdwavelength (e.g., 1565 nm) and passes other wavelengths. In anotherembodiment, each grating has a chirped grating spacing, which serves tocompensate for dispersion experienced by the data stream propagating onthat wavelength.

[0040]FIG. 3 is a schematic diagram of a photonic device 300 accordingto an embodiment of the present invention. The photonic device 300includes several waveguides 302, 304, 306, and 308 formed in or on a PLCplatform 310. The wave-guides 302, 304, 306, and 308 include a grating312, 314, 316, and 318, respectively. The platform 310 is similar to theplatform 104. The waveguides 302, 304, 306, and 308 may be similar tothe waveguide 102.

[0041] The gratings 312, 314, 316, and 318 may be Bragg gratings whosegrating spacings are nominally identical but whose center wavelengthsare different because each grating 312, 314, 316, and 318 has adifferent geometry (e.g., width, depth, height) to give these gratingregions different effective refractive indices. In one embodiment of thepresent invention, the grating 312 may be seven microns wide, thegrating 314 may be six microns wide, the grating 316 may be five micronswide, and the grating 318 may be four microns wide.

[0042] In one embodiment of the present invention, one or more sectionsof the waveguides 302, 304, 306, and 308 are loaded with hydrogen(mentioned above as hydrogenating). Such hydrogen loading improves thephotosensitivity of the waveguide by orders of magnitude and changes theeffective refractive indices of the one or more of the loaded sectionsof waveguides 302, 304, 306, and/or 308. For example, hydrogen may beselectively implanted in the regions of the waveguides 302, 304, 306,and 308 corresponding to the gratings 312, 314, 316, and 318. Thehydrogen can be implanted through ion implantation or localized brushingwith a hydrogen flame. The hydrogen loading will increase thephotosensitivity locally and allow for the generation of differentrefractive indices for similar levels of UV exposure. In thisembodiment, the wavelength strength (reflectivity) and center-wavelengthare directly coupled during UV writing of the Bragg gratings.

[0043] In other embodiments of the present invention, one or moresections of the waveguides 302, 304, 306, and 308 are selectivelypre-exposed or postexposed to different levels of uniform UV radiation.This serves to change the average refractive index of individualwaveguides and shift the centerwavelengths of the gratings 312, 314,316, and 318.

[0044]FIG. 4 is a graphical representation 400 illustrating therelationship between the width of a waveguide and the center wavelengthof a Bragg grating written in the waveguide according to an embodimentof the present invention. The graphical representation 400 includes an“x” axis 402, which represents waveguide width in micrometers (μm), anda “y” axis 404, which represents wavelength in nanometers (nm). Thegraphical representation 400 includes a curve 406, which represents thechange in center wavelength of a Bragg grating written in the waveguideas the width of the waveguide changes. Note that in FIG. 4, as the widthof the waveguide increases, the center wavelength also increases.

[0045] In another embodiment, the gratings 312, 314, 316, and 318 may beBragg gratings whose grating spacings are nominally identical but whosecenter wavelengths are different because the core of one or more of thewaveguides 302, 304, 306, and/or 308 in the grating regions has a beendoped so that the grating regions have different effective refractiveindices. FIG. 5 is a graphical representation 500 illustrating therelationship between the refractive index of a waveguide core and thecenter wavelength of a Bragg grating written in the waveguide accordingto an embodiment of the present invention.

[0046] The graphical representation 500 includes an “x” axis 502, whichrepresents core index of refraction, and a “y” axis 504, whichrepresents wavelength in nanometers. The graphical representation 500includes a curve 506, which represents the change in center wavelengthof a Bragg grating written in the waveguide as the index of refractionof the waveguide changes. Note that in FIG. 5, as the refractive indexof the waveguide increases, the center wavelength also increases. In oneembodiment, the cores may be doped with different concentrations ofdopants according to embodiments of the present invention to vary therefractive index of gratings written in the waveguides. In anotherembodiment, the cores may be doped with different dopant compositions(e.g., aluminum, boron, phosphorous) according to embodiments of thepresent invention to vary the refractive index of gratings written inthe waveguides.

[0047] In still another embodiment, the gratings 312, 314, 316, and 318may be Bragg gratings whose grating spacing are nominally identical butwhose center wavelengths are different because the cladding of thewaveguides 302, 304, 306, and/or 308 in the grating regions havedifferent effective refractive indices. For example, layers of SiO maybe grown on a Si substrate using plasma enhanced chemical vapordeposition (PECVD) techniques. The Si substrate may be nominally fifteenmicrons thick. A lower cladding layer may be deposited on the Sisubstrate under the layers of SiO. A core layer may be formed using theSiO doped with germanium and/or boron, which may increase the refractiveindex of the SiO. The core layer may be six microns thick. Portions ofthe core layer may be wet-etched to leave a pattern for the waveguides302, 304, 306, and/or 308. Germanium, boron, or other suitable dopantsmay be doped in the core layer of the waveguides 302, 304, 306, and/or308 in the grating regions for each grating 312, 314, 316, and 318 usingion implantation, for example. An upper cladding layer may be depositedon the core layer. The upper cladding layer may doped with phosphorousand/or boron. The upper cladding layer may have a thickness of aroundfifteen to twenty microns. The refractive index of the upper claddinglayer is similar to the refractive index of lower cladding layer.

[0048]FIG. 6 is a flowchart illustrating a process 600 for making thephotonic device 300 according to embodiments of the present invention. Amachine-readable medium with machine-readable instructions thereon maybe used to cause a processor to perform the process 600. Of course, theprocess 600 is only an example process and other processes may be used.The order in which the blocks are described should not be construed toimply that these operations are necessarily order-dependent or that theoperations be performed in the order in which the blocks are presented.

[0049] An operation 602 is performed to fabricate waveguides ofdifferent widths in or on a PLC platform using standard semiconductorfabrication techniques. As previously described, these techniquesinclude ion implantation, diffusion doping, evaporation, physical vapordeposition, ion assisted deposition, photolithography, magnetronsputtering, electron beam sputtering, masking, reactive ion etching,and/or other semiconductor fabrication techniques well known to thoseskilled in the art. In one embodiment of the present invention, thewidths of the waveguide regions corresponding to the gratings 312, 314,316 and 318 are seven microns, six microns, five microns, and fourmicrons, respectively.

[0050] An operation 604 is performed to expose the waveguide regionscorresponding to the gratings 312, 314, 316, and/or 318, respectively,to a selected UV light intensity pattern. In this embodiment, the UVlight intensity pattern is provided in a direction transverse to thelongitudinal axes of waveguides 302, 304, 306, and/or 308. This exposurewrites the grating with the desired grating spacing in the regions ofwaveguide 302, 304, 306, and/or 308 for the gratings 312, 314, 316,and/or 318. As previously described, a suitable KrF excimer laser can beused to expose the doped waveguide 302, 304, 306, and 308 regions to theUV light intensity pattern. The UV light intensity pattern may have aheight of 300 microns. In one embodiment, the area occupied by thesewaveguide regions has a width (or height) that is less than 300 microns.Thus, the exposure may write all gratings simultaneously, any one of thegratings individually, or any subset of the gratings simultaneously.

[0051] Devices implemented according to embodiments of the presentinvention may be more compact, simpler to fabricate, and less expensive.For example, in one embodiment, the photonic device 300 may beimplemented as a multiple wavelength division multiplexing (WDM) filterin which one or more of the gratings 312, 314, 316, and 318 isseparately addressable. For example, the device 300 may be aforty-channel cascaded series of channel dispersion compensatingwaveguide gratings having twenty-five gigahertz (GHz) spacing. Thedevice 300 thus may be six centimeters long and two centimeters wide. Inembodiments in which devices are implemented as chirped waveguidegratings, a controlled tapering of the refractive index may greatlyimprove “group delay ripple” that can plague gratings produced by thestandard chirped phase-mask approach.

[0052]FIG. 7 is a schematic diagram of a photonic device 700 accordingto an embodiment of the present invention. The photonic device 700includes several waveguides 702, 704, 706, 708, and 710 formed in or ona PLC platform 750. Each waveguide 702, 704, 706, 708, and 710 includesa grating 712, 714, 716, 718, and 720. The waveguides 702, 704, 706,708, and 710 are similar to the waveguides 302, 304, 306, and 308. Theplatform 750 is similar to the platform 104.

[0053] The gratings 712, 714, 716, 718, and 720 are similar to thegratings 106, 108, and 110 in that the gratings 712, 714, 716, 718, and720 may be Bragg gratings whose grating spacing are nominally identical.The gratings 712, 714, 716, 718, and 720 are similar to the gratings312, 314, 316, and 318 in that the center wavelengths are differentbecause each grating 312, 314, 316, and 318 has a different width togive the grating regions of the waveguides 312, 314, 316, and 318different refractive indices. The gratings 712, 714, 716, 718, and 720are different from the gratings 312, 314, 316, and 318 in that the widthof one or more of the gratings 712, 714, 716, 718 and/or 720 is taperedas shown with respect to the grating 720. As is well known, taperinggives the grating a “chirp” (i.e., a subset of non-uniform refractiveindices along the length of the grating). The chirp may varysymmetrically, asymmetrically, either increasing or decreasing.Alternatively, the chirp may be linear (e.g., the refractive indexvaries linearly with the length of the grating). The chirp may bequadratic, random, or discrete.

[0054] In one embodiment of the present invention, the width of thegrating 720 at points 730, 732, 734 and 736 may be seven microns, sixmicrons, five microns, and four microns, respectively. After reading thedescription herein, persons of ordinary skill in the relevant art(s)will readily recognize how to implement various chirps.

[0055]FIG. 8 is a flowchart illustrating a process 800 for making thephotonic device 700 according to embodiments of the present invention. Amachine-readable medium with machine-readable instructions thereon maybe used to cause a processor to perform the process 800. Of course, theprocess 800 is only an example process and other processes may be used.The order in which the blocks are described should not be construed toimply that these operations are necessarily order-dependent or that theoperations be performed in the order in which the blocks are presented.

[0056] An operation 802 is performed to fabricate waveguides of taperedwidths in or on a PLC platform using standard semiconductor fabricationtechniques, such as implantation, doping, evaporation, physical vapordeposition, ion assisted deposition, photolithography, magnetronsputtering, electron beam sputtering, masking, reactive ion etching,and/or other semiconductor fabrication techniques well known to thoseskilled in the art. In one embodiment of the present invention, thewaveguide 702 may be tapered adiabatically. For example, the width orheight of the waveguide 702 at the points 730, 732, 734 and 736 may beseven microns, six microns, five microns and four microns, respectively.Of course, other shapes or dimensions are possible in other embodiments.

[0057] An operation 804 is performed to expose the waveguide regionscorresponding to the gratings 712, 714, 716, and/or 718 to a selected UVlight intensity pattern in a direction transverse to the waveguides 702,704, 706, 708 and 710. The intensity pattern is generated to write thegrating in the regions of waveguides 702, 704, 706, 708, and 710 for thegratings 712, 714, 716, and/or 718, respectively, with the desiredgating spacing. The exposure may write all gratings simultaneously, anyone of the gratings individually, or any subset of the gratingssimultaneously. In one embodiment, a suitable KrF excimer laser is usedto generate the UV light intensity pattern. The UV light intensitypattern may have a height of 500 microns. In one embodiment, thewaveguide regions corresponding to the gratings 712, 714, 716, and/or718 are closer to each other than 500 microns.

[0058]FIG. 9 is a block diagram of a WDM system 900 using photonicdevices according to embodiments of the present invention. The WDMsystem 900 includes a planar lightwave circuit (PLC) 902 having awaveguide 904 formed therein or thereon and gratings 910, 912, and 914in or on the waveguides 904. These gratings are formed as describedabove. The system 900 also includes an optical signal source 920 thatprovides an optical signal to be received by PLC 902. The gratings 910,912 and 914 provide dispersion compensation across the multiple opticalchannels of the WDM system. After passing through the cascaded gratings910, 912, and 914, the optical signal can be propagated to other opticalcircuitry (not shown). In another embodiment (not shown), PLC 902 mayinclude similar gratings formed in or on separately addressablewaveguides to be used as WDM filters.

[0059] Embodiments of the invention can be implemented using hardware,software, or a combination of hardware and software. In implementationsusing software, the software may be stored on a computer program product(such as an optical disk, a magnetic disk, a floppy disk, etc.) or aprogram storage device (such as an optical disk drive, a magnetic diskdrive, a floppy disk drive, etc.).

[0060] The above description of illustrated embodiments of the inventionis not intended to be exhaustive or to limit embodiments of theinvention to the precise forms disclosed. While specific embodiments of,and examples for, the invention are described herein for illustrativepurposes, various equivalent modifications are possible, as thoseskilled in the relevant art will recognize. These modifications can bemade to embodiments of the invention in light of the above detaileddescription.

[0061] The terms used in the following claims should not be construed tolimit the invention to the specific embodiments disclosed in thespecification and the claims. Rather, the scope of embodiments of theinvention is to be determined entirely by the following claims, whichare to be construed in accordance with established doctrines of claiminterpretation.

What is claimed is:
 1. A method of creating multiple gratings in or on asingle planar lightwave circuit (PLC), comprising: forming a set ofregions having different effective refractive indices in a waveguide;and exposing the set of regions to an ultraviolet (UV) intensity patternto form a set of gratings, wherein the set of gratings have thesubstantially identical gating spacing and different center wavelengths.2. The method of claim 1, wherein the set of regions is exposedsimultaneously.
 3. The method of claim 1, wherein the set of regions hassubstantially identical width profiles.
 4. The method of claim 3,wherein the set of regions have width profiles that vary along alongitudinal axis of the waveguide.
 5. The method of claim 1, whereinforming the set of regions comprises doping the set of different regionsso that each region of the set of regions has a doping parameter that isdifferent from that of the other regions of the set of regions.
 6. Themethod of claim 5, wherein one region of the set of regions is dopedwith a selected dopant at a first concentration and another region ofthe set of regions is doped with the selected dopant at a secondconcentration different from the first concentration.
 7. The method ofclaim 5, wherein after doping, one region of the set of regions includesa dopant that is not included in another region of the set of regions.8. The method of claim 5, wherein a core layer of a region of the set ofregions is doped.
 9. The method of claim 5, wherein a cladding layer ofa region of the set of regions is doped.
 10. The method of claim 1,wherein forming the set of regions comprises forming a first region ofthe set of regions with a geometry that is different from that of asecond region of the set of regions.
 11. The method of claim 10, whereinthe first region has a depth that is different from that of the secondregion.
 12. The method of claim 10, wherein the first region has a widththat is different from that of the second region.
 13. The method ofclaim 1, wherein the set of gratings are chirped.
 14. The method ofclaim 1, wherein the waveguide is arranged in a pattern so that regionsof the set of regions are proximate to each other.
 15. The method ofclaim 1, wherein forming the set of regions includes hydrogenatingselected sections of the set of regions.
 16. The method of claim 15,further comprising pre-exposing the selected sections with light beforehydrogenating the selected sections.
 17. The method of claim 15, whereinion implantation is used to hydrogenate the selected sections.
 18. Aproduct formed by the process of claim
 1. 19. A planar lightwave circuit(PLC), comprising: a first grating having a first center wavelength, thefirst grating having a first effective refractive index and a gratingspacing profile; and a second grating having a second center wavelength,the second grating having a second effective refractive index differentfrom that of the first grating and a grating profile that issubstantially the same as that of the first grating.
 20. The PLC ofclaim 19, wherein the first and second gratings are writtensimultaneously.
 21. The PLC of claim 19, wherein the first and secondgratings are formed in regions of the PLC that have different dopingprofiles.
 22. The PLC of claim 21, wherein the region of the firstgrating is doped with a first dopant at a first concentration and theregion of the second grating is doped with the first dopant at a secondconcentration that is different from the first concentration.
 23. ThePLC of claim 21, wherein the region of the first grating includes adopant that is not included in the region of the second grating.
 24. ThePLC of claim 19, wherein the first and second gratings have differentgeometries.
 25. The PLC of claim 19, wherein the first and secondgratings are chirped.
 26. The PLC of claim 19, wherein the first andsecond gratings form part of a propagation path of a single waveguide.27. The PLC of claim 19, wherein the first and second gratings aredisposed in or on a first waveguide and a second waveguide,respectively, formed in or on the PLC.
 28. A system, comprising: anoptical signal source; a lightwave propagation medium; a planarlightwave circuit (PLC) coupled to the optical signal source through theoptical signal medium; the PLC having: a first grating having a firstcenter wavelength, the first grating having a first effective refractiveindex and a grating spacing profile, and a second grating having asecond center wavelength, the second grating having a second effectiverefractive index different from that of the first grating.
 29. Thesystem of claim 28, wherein the first and second gratings are formed inregions of the PLC that have different doping profiles.
 30. The PLC ofclaim 28, wherein the first and second gratings have differentgeometries so that the first and second refractive indices aredifferent.